No one knows exactly what instruments Columbus took with him on his voyage in 1492. He undoubtedly had an astrolabe and a cross-staff. The astrolabe was devised during the first millennium and Arabian astronomers had perfected it as early as the year 700. It is really the basis of all future instruments of its class,--cross-staff, quadrant, sextant. Some of the most beautiful astrolabes preserved in museums are those made for the Persian astronomers in the sixteenth and seventeenth centuries. Columbus probably used the form devised by Martin Behaim which had been adapted for use at sea about the year 1480. Observations with the astrolabe required three persons, one to hold the instrument plumb by the ring, another to sight the sun and adjust the arm, and the third to read the scale. With these difficulties observations were, of course, far from accurate, but approximate time and latitude could be obtained. Another device was the ring-dial, or universal ring-dial as the old works on navigation called it. This differed from the astrolabe by having adjustable rings with the hours and scales engraved upon them. Both of these instruments are now rare. No original cross-staff is known to the writer in any collection in this country. It consisted of a rod thirty-six inches long on which another of twenty-six inches was centered and arranged to slide up and down at right angles to it. By sighting from the end of the longer rod and moving the sliding bar until the sun was seen at one end of it and the horizon at the other, the figure on the scale at the junction of the rods indicated the sun's altitude and from this the latitude was obtained. [Illustration: SEVENTEENTH CENTURY MARINER USING A CROSS-STAFF From Seller's "Practical Navigation," London, 1676] [Illustration: SEVENTEENTH CENTURY MARINER USING DAVIS' QUADRANT From Seller's "Practical Navigation," London, 1676]